This invention relates to an improved frequency to voltage converter for a digital tachometer of the type used in connection with an incremental measuring system which generates at least two periodic, phase shifted measured signals in response to the relative movement of a first object with respect to a second object.
Tachometers of the type which include direct current tachogenerators that operate to generate signals indicative of rotational velocity according to the dynamo principle are known to the art.
It is also a known practice to provide a digital tachometer which utilizes an incremental rotation transducer which generates two measured signals. These two measured signals are periodic and offset with respect to one another by 90.degree.. They are applied to a direction discriminator circuit which responds to the phase relationship between the two measured signals to generate two pulse trains, a forward pulse train and a backward pulse train. Each of these two pulse trains is applied to a respective switching circuit which generates a sequence of constant energy pulses in response to the associated pulse train. Each sequence of constant energy pulses is filtered by a low pass filter, and the two filtered sequences of pulses are applied to the two inputs of a differential amplifier. The output signal of this differential amplifier is proportional to the rotational velocity sensed by the rotation transducer.
The usable frequency range of such a circuit is limited at high frequencies by the time duration of the constant energy pulses. At low frequencies, the usable frequency range is limited by the maximum allowable periodic component in the output signal of the differential amplifier, which is determined by the time constant of the low pass filters. If the direction discriminator is designed to provide multiple output signals for each cycle of the rotation transducer, and all of these output signals are applied to a single switching circuit, the usable frequency range is shifted, but it is not increased.